Enhanced Thermochromic Performance of VO2 Nanoparticles by Quenching Process

Vanadium dioxide (VO2) has been a promising energy-saving material due to its reversible metal-insulator transition (MIT) performance. However, the application of VO2 films has been seriously restricted due to the intrinsic low solar-energy modulation ability (ΔTsol) and low luminous transmittance (Tlum) of VO2. In order to solve the problems, the surface structure of VO2 particles was regulated by the quenching process and the VO2 dispersed films were fabricated by spin coating. Characterizations showed that the VO2 particles quenched in deionized water or ethanolreserved VO2(M) phase structure and they were accompanied by surface lattice distortion compared to the pristine VO2. Such distortion structure contributed to less aggregation and highly individual dispersion of the quenched particles in nanocomposite films. The corresponding film of VO2 quenched in water exhibited much higher ΔTsol with an increment of 42.5% from 8.8% of the original VO2 film, because of the significant localized surface plasmon resonance (LSPR) effect. The film fabricated from the VO2 quenched in ethanol presented enhanced thermochromic properties with 15.2% of ΔTsol and 62.5% of Tlum. It was found that the excellent Tlum resulted from the highly uniform dispersion state of the quenched VO2 nanoparticles. In summary, the study provided a facile way to fabricate well-dispersed VO2 nanocomposite films and to facilitate the industrialization development of VO2 thermochromic films in the smart window field.


Introduction
Since Morin F. J. discovered the reversible metal-insulator transition (MIT) behavior of vanadium oxides in 1959 [1], VO, V 2 O 3 , VO 2 , V 6 O 13 , V 3 O 7 , and V 2 O 5 [1][2][3] have been reported undergoing MIT at different ambient temperatures and showing significant gaps of the electrical, optical, magnetic properties before and after the MIT. Among them, the phase transition temperature (T c ) of VO 2 , 341 K (68 • C), is the closest to room temperature, which makes VO 2 a promising candidate for intelligent film for smart windows. Below T c , the strong electron-correlated VO 2 is monoclinic (M phase, P2 1 /c) and highly transparent to NIR, which helps warm up the indoor room. When the ambient temperature rises above T c , the VO 2 crystal transits to be tetragonal (R phase, P4 2 /mnm) and reflective to the NIR, which is beneficial for blocking out the thermal radiation and cooling down the room temperature. Such characteristic of temperature-sensitive response of VO 2 is expected to enable intelligent regulation of indoor temperature, thereby reducing the energy consumption of architectures [2][3][4].
However, challenges remain in balancing the admirable luminous transmittance (T lum ) for illuminance and the appreciable solar-energy modulation ability (∆T sol ) for temperature regulations [5][6][7]. Such problems severely hinder the application of VO 2 thermochromic smart windows. Up till now, plenty of strategies have been dedicated to the performance optimization of VO 2 . Element doping is confirmed to be an effective way to diminish the

Fabrication of Quenched VO 2 (M) Nanoparticles
All reagents that were used directly were analytically pure and provided by Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. A certain mass of vanadium pentoxide (V 2 O 5 , 0.8 g) powders and half the amount of ammonium bicarbonate (NH 4 HCO 3 , 0.4 g) particles were loaded together in a quartz crucible and then transferred into the tube furnace. The furnace was pumped below 50 Pa and heated around 550 • C until the powders changed to dark-blue VO 2 particles. After that, the particles were processed with quenching treatment as Scheme 1 described. In detail, VO 2 particles were placed in a vacuum tube furnace for thermal insulation treatment for half an hour and then they were rapidly moved into the 0 • C quenching solution surrounded by an ice-water system. Extra ice bulks were constantly added to the system so that the quenching solvent was maintained at around 0 • C. Next, the beaker holding the above-mentioned solution mixed with quenched VO 2 grains was put in a thermostatic oven to completely evaporate the solvent. Finally, the remaining nanoparticles in the beaker were collected as quenched VO 2 (M).

Fabrication of Quenched VO2(M) Nanoparticles
All reagents that were used directly were analytically pure and provided by Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. A certain mass of vanadium pentoxide (V2O5, 0.8 g) powders and half the amount of ammonium bicarbonate (NH4HCO3, 0.4 g) particles were loaded together in a quartz crucible and then transferred into the tube furnace. The furnace was pumped below 50 Pa and heated around 550 °C until the powders changed to dark-blue VO2 particles. After that, the particles were processed with quenching treatment as Scheme 1 described. In detail, VO2 particles were placed in a vacuum tube furnace for thermal insulation treatment for half an hour and then they were rapidly moved into the 0 °C quenching solution surrounded by an ice-water system. Extra ice bulks were constantly added to the system so that the quenching solvent was maintained at around 0 °C. Next, the beaker holding the above-mentioned solution mixed with quenched VO2 grains was put in a thermostatic oven to completely evaporate the solvent. Finally, the remaining nanoparticles in the beaker were collected as quenched VO2(M). Scheme 1. Diagram of the quenching process of the VO2 particles.

Fabrication of VO2-PVP Nanocomposite Film
The obtained VO2(M) nanoparticles were mixed with polyvinylpyrrolidone (PVP, K30) and ethanol in the mill tank. Intermittent ball milling gave rise to particles with smaller sizes to achieve thorough dispersion in the ethanol. Afterward, the mixture liquid was transferred into a centrifuge tube for solid-liquid separation. The upper liquid in the tube remained turbid as VO2 with extreme fine sizes dispersed well in PVP, which dissolved well in ethanol. With the evaporation of liquid in the upper suspension, VO2-PVP nanocomposites were collected and then made into a coating slurry by mixing them with additional ethanol. Continuous stirring and ultrasonic vibration were implemented to make the slurry homogeneous. Then spin-coating was carried out to form a sol/wet film by dropping the solution on a common soda-lime-silica glass substrate. In the end, ethanol was removed by heating to form the VO2-PVP nanocomposite films.

Characterization
X-ray diffraction (XRD, D8DISCOVER, Bruker, Billerica, MA, USA) with Cu Kα (λ = 0.154056 nm) serving as the source of radiation, and 3 kW of the output power)was adopted to determine the phase structures of the powders over the 2θ between 10° and 80°. A differential scanning calorimeter (DSC, DSC8500,PerkinElmer, Waltham, MA, USA) was used to examine the phase transition temperature of the powders with the temperature ranging from 0 °C to 100 °C at the rate of 5 °C/min in the heating/cooling loop, as illustrated in Equation (1). Tc refers to the average phase transition temperature of VO2 particles. Tc,h and Tc,c correspond to the phase transition temperature peak of VO2 in the heating and cooling stages, respectively. A field emission scanning electron microscopy (FE-SEM, Zeiss Ultra Plus, Carl Zeiss CMP GmbH, Oberkochen, Germany) was used to observe the morphology of both the composite powders and films. X-ray photoelectron Scheme 1. Diagram of the quenching process of the VO 2 particles.

Fabrication of VO 2 -PVP Nanocomposite Film
The obtained VO 2 (M) nanoparticles were mixed with polyvinylpyrrolidone (PVP, K30) and ethanol in the mill tank. Intermittent ball milling gave rise to particles with smaller sizes to achieve thorough dispersion in the ethanol. Afterward, the mixture liquid was transferred into a centrifuge tube for solid-liquid separation. The upper liquid in the tube remained turbid as VO 2 with extreme fine sizes dispersed well in PVP, which dissolved well in ethanol. With the evaporation of liquid in the upper suspension, VO 2 -PVP nanocomposites were collected and then made into a coating slurry by mixing them with additional ethanol. Continuous stirring and ultrasonic vibration were implemented to make the slurry homogeneous. Then spin-coating was carried out to form a sol/wet film by dropping the solution on a common soda-lime-silica glass substrate. In the end, ethanol was removed by heating to form the VO 2 -PVP nanocomposite films.

Characterization
X-ray diffraction (XRD, D8DISCOVER, Bruker, Billerica, MA, USA) with Cu Kα (λ = 0.154056 nm) serving as the source of radiation, and 3 kW of the output power)was adopted to determine the phase structures of the powders over the 2θ between 10 • and 80 • . A differential scanning calorimeter (DSC, DSC8500,PerkinElmer, Waltham, MA, USA) was used to examine the phase transition temperature of the powders with the temperature ranging from 0 • C to 100 • C at the rate of 5 • C/min in the heating/cooling loop, as illustrated in Equation (1). T c refers to the average phase transition temperature of VO 2 particles. T c,h and T c,c correspond to the phase transition temperature peak of VO 2 in the heating and cooling stages, respectively. A field emission scanning electron microscopy (FE-SEM, Zeiss Ultra Plus, Carl Zeiss CMP GmbH, Oberkochen, Germany) was used to observe the morphology of both the composite powders and films. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, ThermoFisher, Waltham, MA, USA) was utilized to determine the element composition and valence. FT−IR spectrum (Nicolet6700, ThermoFisher, Waltham, MA, USA) was served to identify the functional groups of the samples.
The thermochromic performance of the films was measured from 300 to 2500 nm by a UV-vis-NIR spectrophotometer (UV-3600) equipped with a temperature-controlling device. The transmittance of the films at 20 • C and 90 • C corresponded to VO 2 (M) and VO 2 (R), respectively. The integrated luminous transmittance (T lum , 380 nm ≤ λ ≤ 780 nm) and solar transmittance (T sol , 300 nm ≤ λ ≤ 2500 nm) can be calculated according to Equations (2) and (3).
In the equations, T(λ) represents the film transmittance of light at a certain wavelength (λ), ϕ lum (λ) is the standard luminous efficiency function for the photopic vision of human eyes [11], and ϕ sol (λ) is the solar irradiance spectrum for air mass 1.5 corresponding to the sun standing 37 • above the horizon. ∆T sol is the difference value of T sol at 20 • C and 90 • C, as given in Equation (4).

Results and Discussion
3.1. Structure of the VO 2 Nanoparticles VO 2 powders were obtained from V 2 O 5 by a facial annealing reduction reaction. As shown in the TG-DSC pattern (Figure 1a) of the mixture of V 2 O 5 and NH 4 HCO 3 , NH 4 HCO 3 kept decomposing to release NH 3 at the beginning during the homogeneous heating period, illustrating the continuous mass loss of the raw materials. Till 183.5 • C, NH 4 HCO 3 decomposed completely and the mass loss in this period was 8.28% in total. The endothermic peak at 226.2 • C indicated that the absorbed H 2 O was released, accounting for 3.29% of the reaction agents. With the rising temperature, the reducibility of NH 3 contributed to changing the vanadium (V) in V 2 O 5 to lower valence, accompanied by an exothermic process at 360.0 • C, and finally reached the stable phase until 400 • C, as no mass loss or well as energy exchange could be observed in the pattern. This result suggested that 400 • C was suitable for V 2 O 5 reduction. Additionally, the small temperature deviation between DSC and DTG could be attributed to the errors caused by the test instrument. Figure 1b Figure 1a. The reason could be attributed to the fact that the furnace chamber in the annealing process was too large, and the effective heating interval of the furnace was small, which caused partial energy loss. When raising the annealing temperature to 450 • C, the fabricated samples came across the same situation. Interestingly, the diffraction peaks belonging to V 6 O 13 of the VO 2 sample reduced at 450 • C were much weaker than those of VO 2 reduced at 400 • C, testifying that the proportion of V 6 O 13 among the VO 2 samples annealed at 450 • C was much less than that of the VO 2 annealed at 400 • C. Thus, a higher annealing temperature was demanded to overcome the energy barrier for reducing V 2 O 5 into VO 2 thoroughly. In addition, it turned out that the material annealed at 500 • C became pure VO 2 (M) with sharp diffraction peaks. Such peaks implied that the 500 • C reduced VO 2 featured excellent crystallinity. Comparably, the powders annealed at 550 • C remained pure VO 2 (M) structure but exhibited a decreased peak intensity, indicating worse crystallinity of the related VO 2 powders. This might be caused by the unexpected grain agglomeration during the thermal insulation at higher temperatures. Therefore, 500 • C was selected as the ideal temperature to produce pristine VO 2 (labeled as P-VO 2 ). Nanomaterials 2023, 13, x FOR PEER REVIEW 5 of 14 was demanded to overcome the energy barrier for reducing V2O5 into VO2 thoroughly. In addition, it turned out that the material annealed at 500 °C became pure VO2(M) with sharp diffraction peaks. Such peaks implied that the 500 °C reduced VO2 featured excellent crystallinity. Comparably, the powders annealed at 550 °C remained pure VO2(M) structure but exhibited a decreased peak intensity, indicating worse crystallinity of the related VO2 powders. This might be caused by the unexpected grain agglomeration during the thermal insulation at higher temperatures. Therefore, 500 °C was selected as the ideal temperature to produce pristine VO2 (labeled as P-VO2). To observe the metal-insulator transition, P-VO2 was characterized through in-situ XRD technique, and the results were shown in Figure 2a,b, and Figure 2b was the enlarged view of the pink area in Figure 2a. The VO2 stayed the monoclinic structure below 45 °C, and the main peak (27.76°) corresponded to the (011) crystal plane of VO2(M). When the temperature rose above 65 °C, the peak shifted to 27.63°, revealing that the VO2 crystals had transferred to the rutile phase, featured with the (110) crystal plane of VO2(R) (PDF #73-2362). This phenomenon suggested that the phase transition temperature of P-VO2 ranged between 45 °C and 65 °C. This was in agreement with previous reports [5] for VO2 particles. When the VO2 particles cooled down from 75 °C to 65 °C, VO2 crystals transited back to monoclinic structure from rutile structure. This helped obverse the evident and reversible MIT behavior between VO2(M) and VO2(R) in the previous report [34]. Furthermore, the phase transition temperature of the cooling stage was inconsistent with that of the heating stage, confirming the thermal hysteresis loop feature of VO2 in the phase transition stage. It was the MIT behavior that made VO2 a promising material for smart windows. To observe the metal-insulator transition, P-VO 2 was characterized through in-situ XRD technique, and the results were shown in Figure 2a,b, and Figure 2b was the enlarged view of the pink area in Figure 2a. The VO 2 stayed the monoclinic structure below 45 • C, and the main peak (27.76 • ) corresponded to the (011) crystal plane of VO 2 (M). When the temperature rose above 65 • C, the peak shifted to 27.63 • , revealing that the VO 2 crystals had transferred to the rutile phase, featured with the (110) crystal plane of VO 2 (R) (PDF #73-2362). This phenomenon suggested that the phase transition temperature of P-VO 2 ranged between 45 • C and 65 • C. This was in agreement with previous reports [5] for VO 2 particles. When the VO 2 particles cooled down from 75 • C to 65 • C, VO 2 crystals transited back to monoclinic structure from rutile structure. This helped obverse the evident and reversible MIT behavior between VO 2 (M) and VO 2 (R) in the previous report [34]. Furthermore, the phase transition temperature of the cooling stage was inconsistent with that of the heating stage, confirming the thermal hysteresis loop feature of VO 2 in the phase transition stage. It was the MIT behavior that made VO 2 a promising material for smart windows. In order to cause surface distortion for the crystal, P-VO2 featured with the notable MIT behavior was placed in a 0 °C ice-water system for quenching treatment as Scheme 1 illustrated. According to the difference in quenching solvent, the VO2 quenched in deionized water was named H-VO2 and the one quenched in ethanol was noted as E-VO2. The XRD pattern of VO2 under different quenching solvents was shown in Figure 3a. Compared to the pristine VO2 without the quenching process, both the H-VO2 and E-VO2 pow- In order to cause surface distortion for the crystal, P-VO 2 featured with the notable MIT behavior was placed in a 0 • C ice-water system for quenching treatment as Scheme 1 il-Nanomaterials 2023, 13, 2252 6 of 13 lustrated. According to the difference in quenching solvent, the VO 2 quenched in deionized water was named H-VO 2 and the one quenched in ethanol was noted as E-VO 2 . The XRD pattern of VO 2 under different quenching solvents was shown in Figure 3a. Compared to the pristine VO 2 without the quenching process, both the H-VO 2 and E-VO 2 powders held the phase structure of VO 2 (M) in Figure 1b. A smaller peak located at 25.3 • appeared in the quenched samples, which could be the localized oxidation caused by exposure to air during powder transfer. It is worth noting that quenched particles presented the VO 2 (M) phase and manifested great crystallinity. In addition, surface lattice distortion of the VO 2 was assumed to occur after the quenching process [35], which would be discussed in the next part. According to the Scherrer equation, the average grain size of E-VO 2 and H-VO 2 were calculated as 30.08 nm and 17.06 nm, respectively, larger than the average size (12.11 nm) of P-VO 2 . To explain the mean grain size changes of the three samples, an enlarged view of the XRD diffraction angle ranging from 27.0 • to 28.5 • (Organe area in Figure 3a) was presented to illustrate the half-width changes of the peaks. As shown in Figure 3b, the diffraction curve of P-VO 2 exhibited the greatest widening state compared to E-VO 2 and H-VO 2 . Such a state endowed P-VO 2 with the largest half-width and thus, the smallest grain size among the three particles. On the other hand, the half-width of the E-VO 2 (011) crystal plane diffraction peak was approaching that of H-VO 2 . However, the diffraction peak of E-VO 2 shifted to a higher position at 27.91 • , compared to the peak at 27.86 • of P-VO 2 . Such a shift to a higher angle caused the fact that the grain size of E-VO 2 was a bit larger than that of H-VO 2 . This phenomenon might be attributed to the hydroxy group attached to the surface of VO 2 when the heated particles encountered an ice-water/ice-ethanol system and thus, leading to surface lattice distortion of the crystals. Moreover, thermal insulation during the quenching process in Scheme 1 was also beneficial to grain growth.  With the goal of further determining the effect of the quenching process on VO2, the XPS technique was used to identify the chemical environment of elements, and the results were presented in Figure 4. Regarding the C1s peak at 284.80 eV as the calibration position, the full spectrum (Figure 4a) illustrated the existence of vanadium and oxygen in all the P-VO2, H-VO2, and E-VO2 powders according to the V2p and O1s peaks. The V2p peaks were analyzed by Avantage of Thermo Scientific. In addition, Shirley background subtraction was employed to diminish the influence of heightened peaks due to signals from electrons undergoing inelastic scattering during the XPS characterization, ensuring a convincing quantification analysis of the peaks. As the high-resolution pattern for V2p shown in Figures 4b to d, both the V2p3/2 and V2p1/2 spectral peaks were significantly asymmetrical and each was split into two peaks, which implied that the vanadium of all the VO2 samples involved two different chemical states, corresponding to V 4+ (The fitted purple curve) and V 5+ (The fitted green curve), respectively. V 5+ owing to V2O5 consisted of V2p3/2 at 517.5 eV and V2p1/2 at 525.0 eV. The gap between the V2p3/2 orbital and V2p3/2 orbital was 7.5 eV, which is in line with previous work [13]. Additionally, it is supposed that the existence of V 5+ was caused by partial oxidation when the VO2 particles were ex- With the goal of further determining the effect of the quenching process on VO 2 , the XPS technique was used to identify the chemical environment of elements, and the results were presented in Figure 4. Regarding the C1s peak at 284.80 eV as the calibration position, the full spectrum (Figure 4a) illustrated the existence of vanadium and oxygen in all the P-VO 2 , H-VO 2, and E-VO 2 powders according to the V2p and O1s peaks. The V2p peaks were analyzed by Avantage of Thermo Scientific. In addition, Shirley background subtraction was employed to diminish the influence of heightened peaks due to signals from electrons undergoing inelastic scattering during the XPS characterization, ensuring a convincing quantification analysis of the peaks. As the high-resolution pattern for V2p shown in Figure 4b to d, both the V2p 3/2 and V2p 1/2 spectral peaks were significantly asymmetrical and each was split into two peaks, which implied that the vanadium of all the VO 2 samples involved two different chemical states, corresponding to V 4+ (The fitted purple curve) and V 5+ (The fitted green curve), respectively. V 5+ owing to V 2 O 5 consisted of V2p 3/2 at 517.5 eV and V2p 1/2 at 525.0 eV. The gap between the V2p 3/2 orbital and V2p 3/2 orbital was 7.5 eV, which is in line with previous work [13]. Additionally, it is supposed that the existence of V 5+ was caused by partial oxidation when the VO 2 particles were exposed to air [36]. The V 4+ attributed to VO 2 was made up of V2p 3/2 at 516.2 eV and V2p 1/2 at 523.7 eV. In addition, the V2p 3/2 of P-VO 2 was most occupied with V 5+ , and V 4+ only accounted for a small proportion. Comparably, the V 4+ in the V2p orbitals of H-VO 2 in Figure 4c showed the highest percentage, followed by E-VO 2 in Figure 4d, and finally, the P-VO 2 declared the least V 4+ content in Figure 4b. Such results could be attributed to the surface lattice distortion of H-VO 2 and E-VO 2 .  SEM was employed to investigate the morphology of the aforementioned VO2 particles. Due to the large size of the commercial precursor V2O5, the directly annealed P-VO2 particles inherited the large grain size, as shown in Figure 5a. It is evident that the particles aggregated severely to form large-scale clusters, with irregular shapes and different sizes ranging from 0.2 μm to 1.2 μm.Unfortunately, such sizes hindered P-VO2 from dispersing in the PVP matrix for film coating. Compared to the pristine sample, the degree of particle aggregation of quenched VO2 was much improved. In Figure 5b, the big clusters were broken into small H-VO2 parts with varying sizes, showing that the deionized water was able to split the large-size P-VO2 gathering during the quenching process. Especially, when the quenched solvent was replaced with ethanol with a smaller density, E-VO2 interacted more severely with the liquid and turned more entire separation into nanoparticles with approximately 200 nm in size, as presented in Figure 5c. In addition, the voids clearly appeared among the E-VO2 nanoparticles and the linkage knot hinted the E-VO2 separated from large-size P-VO2 particles. The observation that the quenched VO2 nanoparticles were easily separated into small pellets from large-sized clusters resulted from the thermodynamically unstable state of these aggregated clusters. This unstable state was SEM was employed to investigate the morphology of the aforementioned VO 2 particles. Due to the large size of the commercial precursor V 2 O 5 , the directly annealed P-VO 2 particles inherited the large grain size, as shown in Figure 5a. It is evident that the particles aggregated severely to form large-scale clusters, with irregular shapes and different sizes ranging from 0.2 µm to 1.2 µm.Unfortunately, such sizes hindered P-VO 2 from dispersing in the PVP matrix for film coating. Compared to the pristine sample, the degree of particle aggregation of quenched VO 2 was much improved. In Figure 5b, the big clusters were broken into small H-VO 2 parts with varying sizes, showing that the deionized water was able to split the large-size P-VO 2 gathering during the quenching process. Especially, when the quenched solvent was replaced with ethanol with a smaller density, E-VO 2 interacted more severely with the liquid and turned more entire separation into nanoparticles with approximately 200 nm in size, as presented in Figure 5c. In addition, the voids clearly appeared among the E-VO 2 nanoparticles and the linkage knot hinted the E-VO 2 separated from large-size P-VO 2 particles. The observation that the quenched VO 2 nanoparticles were easily separated into small pellets from large-sized clusters resulted from the thermodynamically unstable state of these aggregated clusters. This unstable state was due to the surface crystal distortion caused by the quenching process, confirming the results of XPS spectrums. It is worth mentioning that the particle was much larger than the value calculated in Figure 3a. The fact that the XRD resulted from Cu Kα radiation reflected crystalline particles rather than the actual morphology of the powders might explain the difference in the mean particle sizes. As for SEM, the signals of secondary electrons with much smaller De Broglie wavelengths were collected to reflect the topography of the particles. The different wavelengths of Cu Kα in XRD and secondary electrons in SEM caused different imaging results. On the other hand, VO 2 particles in Figure 5 were assumed to be polycrystalline consisting of substantial crystals. SEM caused different imaging results. On the other hand, VO2 particles in Figure 5 were assumed to be polycrystalline consisting of substantial crystals. The DSC curves of the above P-VO2 H-VO2 and E-VO2 powders are illustrated in Figure 6a with expected exothermic and endothermic peaks. Such a thermal energy change suggested the phase transition behavior between VO2(M) and VO2(R). The exothermic peak (Tc,h) at 68.35 °C and endothermic peak (Tc,c) at 61.09 °C of P-VO2 implied the average Tc was 64.72 °C according to Equation (1). On the other hand, the energy involved in the MIT behavior was 47 J/g, approaching 51 J/g of the bulk VO2, showing good crystallinity of P-VO2 powders as shown in Figure 2. With the quenching process, the H-VO2 powders with phase transition peaks at 65.61 °C and 58.53 °C presented the average Tc as 62.07 °C, while the Tc of E-VO2 nanoparticles was calculated as 62.73 °C from the Tc,h peak at 65.73 °C and Tc,c peak at 58.24 °C. It is clear that the Tc of the quenched samples was slightly lowered, which could be attributed to the surface lattice distortion of the VO2 crystal structure as the XPS results suggested.
Considering the similar structure of H2O and C2H6O, FT−IR was employed to detect the -OH bond of the VO2 particles. In Figure 6b, the -OH bond corresponded to the peaks at 3450 cm −1 and hydrogen bonds appeared at 1630 cm −1 . The integral area of the peak at 1630 cm −1 was used to reflect the relative content of the hydroxyl group in the samples. It turned out that the hydroxyl peak area of H-VO2 particles was the largest at 106.4, followed by the peak area of 87.7 of E-VO2, and finally the value of 25.3 of P-VO2. Such results indicated that the absorbed hydroxyl on the surface of both H-VO2 and E-VO2 particles was more than P-VO2. This could be due to the surface lattice distortion of the quenched samples, so there were more absorption sites bound to the hydroxyl group on the surface of these particles. Additionally, the peaks at around 990 cm −1 and 890 cm −1 , respectively, corresponded to the stretching vibration and asymmetric stretching vibration of the V=O bond. The overlapping peaks at 720 cm −1 and 660 cm −1 were the characteristic peaks of VO2(M) [37]. In addition, the peaks at around 530 cm −1 were labeled as the stretching vibration of the V-O-V bond. Apart from the peak offset of the corresponding bonds in VO2, small extra peaks at around 1285 m −1 were discovered in H-VO2 and E-VO2, which was attributed to the O-H band after the quenching process. Such a phenomenon confirmed the surface lattice distortion to VO2 crystals of H-VO2 and E-VO2. The DSC curves of the above P-VO 2 H-VO 2 and E-VO 2 powders are illustrated in Figure 6a with expected exothermic and endothermic peaks. Such a thermal energy change suggested the phase transition behavior between VO 2 (M) and VO 2 (R). The exothermic peak (T c,h ) at 68.35 • C and endothermic peak (T c,c ) at 61.09 • C of P-VO 2 implied the average T c was 64.72 • C according to Equation (1). On the other hand, the energy involved in the MIT behavior was 47 J/g, approaching 51 J/g of the bulk VO 2 , showing good crystallinity of P-VO 2 powders as shown in Figure 2. With the quenching process, the H-VO 2 powders with phase transition peaks at 65.61 • C and 58.53 • C presented the average T c as 62.07 • C, while the T c of E-VO 2 nanoparticles was calculated as 62.73 • C from the T c,h peak at 65.73 • C and T c,c peak at 58.24 • C. It is clear that the T c of the quenched samples was slightly lowered, which could be attributed to the surface lattice distortion of the VO 2 crystal structure as the XPS results suggested.
Considering the similar structure of H 2 O and C 2 H 6 O, FT−IR was employed to detect the -OH bond of the VO 2 particles. In Figure 6b, the -OH bond corresponded to the peaks at 3450 cm −1 and hydrogen bonds appeared at 1630 cm −1 . The integral area of the peak at 1630 cm −1 was used to reflect the relative content of the hydroxyl group in the samples. It turned out that the hydroxyl peak area of H-VO 2 particles was the largest at 106.4, followed by the peak area of 87.7 of E-VO 2 , and finally the value of 25.3 of P-VO 2 . Such results indicated that the absorbed hydroxyl on the surface of both H-VO 2 and E-VO 2 particles was more than P-VO 2 . This could be due to the surface lattice distortion of the quenched samples, so there were more absorption sites bound to the hydroxyl group on the surface of these particles. Additionally, the peaks at around 990 cm −1 and 890 cm −1 , respectively, corresponded to the stretching vibration and asymmetric stretching vibration of the V=O bond. The overlapping peaks at 720 cm −1 and 660 cm −1 were the characteristic peaks of VO 2 (M) [37]. In addition, the peaks at around 530 cm −1 were labeled as the stretching vibration of the V-O-V bond. Apart from the peak offset of the corresponding bonds in VO 2 , small extra peaks at around 1285 m −1 were discovered in H-VO 2 and E-VO 2 , which was attributed to the O-H band after the quenching process. Such a phenomenon confirmed the surface lattice distortion to VO 2 crystals of H-VO 2 and E-VO 2 .

Thermochromic Properties and Morphology of VO2 Nanocomposite Films
To reduce the grain size of the as-synthesized particles, VO2 samples were mixed with PVP and ethanol in the mill tank to conduct thorough ball milling and centrifugation, ending up with a dark liquid mixture. The turbid upper solution was dried to collect the VO2-PVP compound, which was then mixed with additional ethanol to configure the spincoating slurry. The slurry was then dropped on the soda-lime-silica glass substrate and spun to form the VO2 nanocomposite films. A UV3600 spectrophotometer coupled with a temperature-controlling device was served to characterize the transmittance of the films ranging from 300 nm to 2500 nm, and the thermochromic properties of the VO2 nanocomposite films were integrated by Equations (2) to (4). The solid lines in Figure 7a were tested at 20 °C, revealing the transmittance of M phase VO2 samples, and the dash lines were obtained at 90 °C, appearing the transmittance of R phase VO2 samples. As shown in Figure 7a and Table 1, the Tlum of P-VO2 film was 53.2% and ΔTsol just reached 8.8%. The grain accumulation in Figure 5a was the cause for such poor properties. Comparably, the H-VO2 film sacrificed a small amount of luminous transmittance of 3.1% to achieve as enormous as 42.5% of improvement in solar-energy modulation ability to reach 12.5%. The reason for the enhanced ΔTsol originated from the LSPR absorption peak of H-VO2 located at 1258 nm, which was stronger than the P-VO2 peak at 1293 nm. Such a phenomenon led to an enlarged gap in the transmittance of VO2 at 20 °C and 90 °C and contributed to higher ΔTsol. The E-VO2 nanocomposite film that came across with the same LSPR effect at 1150 nm revealed an exceeding increment of ΔTsol of 72.2% (from 8.8% to 15.2%). In addition, the Tlum of E-VO2 film was boosted to 62.5%, indicating a better dispersity of the VO2 nanoparticles in the PVP matrix. The optimized luminous transmittance was also credited for less aggregation of the nanoparticles as shown in Figure 5c. In Figure 7b, the thermochromic performance of E-VO2 in this work was compared to previously reported VO2 nanocomposite films, and it was clear that the E-VO2 film exceeded most VO2 films [21,23,27,[31][32][33]. Besides, the excellent ΔTsol of the E-VO2 film was able to satisfy the demands of effectively regulating room temperature while the great Tlum met the requirement of indoor brightness, which was a great achievement for the potential application of VO2 thermochromic smart windows.

Thermochromic Properties and Morphology of VO 2 Nanocomposite Films
To reduce the grain size of the as-synthesized particles, VO 2 samples were mixed with PVP and ethanol in the mill tank to conduct thorough ball milling and centrifugation, ending up with a dark liquid mixture. The turbid upper solution was dried to collect the VO 2 -PVP compound, which was then mixed with additional ethanol to configure the spin-coating slurry. The slurry was then dropped on the soda-lime-silica glass substrate and spun to form the VO 2 nanocomposite films. A UV3600 spectrophotometer coupled with a temperature-controlling device was served to characterize the transmittance of the films ranging from 300 nm to 2500 nm, and the thermochromic properties of the VO 2 nanocomposite films were integrated by Equations (2) to (4). The solid lines in Figure 7a were tested at 20 • C, revealing the transmittance of M phase VO 2 samples, and the dash lines were obtained at 90 • C, appearing the transmittance of R phase VO 2 samples. As shown in Figure 7a and Table 1, the T lum of P-VO 2 film was 53.2% and ∆T sol just reached 8.8%. The grain accumulation in Figure 5a was the cause for such poor properties. Comparably, the H-VO 2 film sacrificed a small amount of luminous transmittance of 3.1% to achieve as enormous as 42.5% of improvement in solar-energy modulation ability to reach 12.5%. The reason for the enhanced ∆T sol originated from the LSPR absorption peak of H-VO 2 located at 1258 nm, which was stronger than the P-VO 2 peak at 1293 nm. Such a phenomenon led to an enlarged gap in the transmittance of VO 2 at 20 • C and 90 • C and contributed to higher ∆T sol . The E-VO 2 nanocomposite film that came across with the same LSPR effect at 1150 nm revealed an exceeding increment of ∆T sol of 72.2% (from 8.8% to 15.2%). In addition, the T lum of E-VO 2 film was boosted to 62.5%, indicating a better dispersity of the VO 2 nanoparticles in the PVP matrix. The optimized luminous transmittance was also credited for less aggregation of the nanoparticles as shown in Figure 5c. In Figure 7b, the thermochromic performance of E-VO 2 in this work was compared to previously reported VO 2 nanocomposite films, and it was clear that the E-VO 2 film exceeded most VO 2 films [21,23,27,[31][32][33]. Besides, the excellent ∆T sol of the E-VO 2 film was able to satisfy the demands of effectively regulating room temperature while the great T lum met the requirement of indoor brightness, which was a great achievement for the potential application of VO 2 thermochromic smart windows.  The morphology of the films was presented. It is clear that P-VO2 turned into nano particles with a size of around 100 nm after a ball milling process as described in Figur 8a. However, particle aggregation in Figure 5a was still common even though they wer spin-coated to film. This aggregation structure was the reason for the unsatisfied thermo chromic properties of the P-VO2 film. Comparably, grain accumulation also existed in H VO2 film, which caused low luminous transmittance of the film. Additionally, cracks could be found in the film, and they were the cause of the inadequate performance of Tlum. It i worth mentioning that most H-VO2 nanoparticles exhibited much smaller sizes than P VO2 and they were dispersed individually in the PVP in Figure 8b, causing the LSPR effec and the improvement of ΔTsol. In Figure 8c, the E-VO2 nanoparticles showed an averag size of tens of nanometers and nearly no sign of particle aggregation. Additionally, thes particles with surface lattice distortion were highly isolated from each other and uni formly dispersed in the E-VO2 nanocomposite film. The large-scale surface morphology of E-VO2 film in Figure 8d introduced such dispersity of VO2 nanoparticles in a more in tuitive perspective. This dispersion structure became beneficial for the optical propertie of E-VO2 nanocomposite film.  The morphology of the films was presented. It is clear that P-VO 2 turned into nanoparticles with a size of around 100 nm after a ball milling process as described in Figure 8a. However, particle aggregation in Figure 5a was still common even though they were spincoated to film. This aggregation structure was the reason for the unsatisfied thermochromic properties of the P-VO 2 film. Comparably, grain accumulation also existed in H-VO 2 film, which caused low luminous transmittance of the film. Additionally, cracks could be found in the film, and they were the cause of the inadequate performance of T lum . It is worth mentioning that most H-VO 2 nanoparticles exhibited much smaller sizes than P-VO 2 and they were dispersed individually in the PVP in Figure 8b, causing the LSPR effect and the improvement of ∆T sol . In Figure 8c, the E-VO 2 nanoparticles showed an average size of tens of nanometers and nearly no sign of particle aggregation. Additionally, these particles with surface lattice distortion were highly isolated from each other and uniformly dispersed in the E-VO 2 nanocomposite film. The large-scale surface morphology of E-VO 2 film in Figure 8d introduced such dispersity of VO 2 nanoparticles in a more intuitive perspective. This dispersion structure became beneficial for the optical properties of E-VO 2 nanocomposite film. Nanomaterials 2023, 13, x FOR PEER REVIEW 12 of 14

Conclusions
As a strong electronic associated material, VO2(M) underwent a reversible phase transition between the monoclinic phase and rutile phase, corresponding with an abrupt change of near-infrared light transmittance, thus showing great potential in the smart window application. To improve the thermochromic properties of VO2, this paper applied a facile annealing method to synthesize VO2(M) powders. With the quenching treatment, the VO2 particles presented surface lattice distortion and they were individually dispersed in the PVP host to induce the LSPR effect, contributing to the exciting increment of solarenergy modulation ability. Hereby, the film corresponding to H-VO2 achieved an enormous improvement in ΔTsol from 8.8% to 12.5%. Moreover, the film fabricated by the ethanol-quenched VO2 with no sign of aggregation showed an exceedingly high ΔTsol of 15.2%, and the high dispersity of E-VO2 nanoparticles in the film contributed to much enhanced Tlum of 62.5%. Therefore, this work provided a new thought to promote the thermochromic performance of VO2 by particle quenching and the strategy was beneficial to the commercialization of VO2 thermochromic smart windows.
Author Contributions: S.W. contributed to writing the original draft and conducting the experiment and data analysis. L.Z. contributed to characterizing the structure of the samples and B.L. was responsible for the optical performance characterization of the films. S.T. contributed to analyzing all the data and supervising the experimental routine and data analysis. X.Z. was involved in analyzing all the data and revising the manuscript. All authors have read and agreed to the published version of the manuscript.

Conclusions
As a strong electronic associated material, VO 2 (M) underwent a reversible phase transition between the monoclinic phase and rutile phase, corresponding with an abrupt change of near-infrared light transmittance, thus showing great potential in the smart window application. To improve the thermochromic properties of VO 2 , this paper applied a facile annealing method to synthesize VO 2 (M) powders. With the quenching treatment, the VO 2 particles presented surface lattice distortion and they were individually dispersed in the PVP host to induce the LSPR effect, contributing to the exciting increment of solar-energy modulation ability. Hereby, the film corresponding to H-VO 2 achieved an enormous improvement in ∆T sol from 8.8% to 12.5%. Moreover, the film fabricated by the ethanol-quenched VO 2 with no sign of aggregation showed an exceedingly high ∆T sol of 15.2%, and the high dispersity of E-VO 2 nanoparticles in the film contributed to much enhanced T lum of 62.5%. Therefore, this work provided a new thought to promote the thermochromic performance of VO 2 by particle quenching and the strategy was beneficial to the commercialization of VO 2 thermochromic smart windows.
Author Contributions: S.W. contributed to writing the original draft and conducting the experiment and data analysis. L.Z. contributed to characterizing the structure of the samples and B.L. was responsible for the optical performance characterization of the films. S.T. contributed to analyzing all the data and supervising the experimental routine and data analysis. X.Z. was involved in analyzing all the data and revising the manuscript. All authors have read and agreed to the published version of the manuscript.